Molecular cloning and characterization of a novel human G-protein-coupled receptor, EDG7, for lysophosphatidic acid

Lysophosphatidylcholine activates adipocyte glucose uptake and lowers blood glucose levels in murine models of diabetes

Glucose homeostasis is maintained by the orchestration of peripheral glucose utilization and hepatic glucose production, mainly by insulin. In this study, we found by utilizing a combined parallel chromatography mass profiling approach that lysophosphatidylcholine (LPC) regulates glucose levels. LPC was found to stimulate glucose uptake in 3T3-L1 adipocytes dose- and time-dependently, and this activity was found to be sensitive to variations in acyl chain lengths and to polar head group types in LPC. Treatment with LPC resulted in a significant increase in the level of GLUT4 at the plasma membranes of 3T3-L1 adipocytes. Moreover, LPC did not affect IRS-1 and AKT2 phosphorylations, and LPC-induced glucose uptake was not influenced by pretreatment with the PI 3-kinase inhibitor LY294002. However, glucose uptake stimulation by LPC was abrogated both by rottlerin (a protein kinase Cdelta inhibitor) and by the adenoviral expression of dominant negative protein kinase Cdelta. In line with its determined cellular functions, LPC was found to lower blood glucose levels in normal mice. Furthermore, LPC improved blood glucose levels in mouse models of type 1 and 2 diabetes. These results suggest that an understanding of the mode of action of LPC may provide a new perspective of glucose homeostasis.

Lysophosphatidic Acid signals through specific lysophosphatidic Acid receptor subtypes to control key regenerative responses of human gingival and periodontal ligament fibroblasts

BACKGROUND

We showed that the pluripotent platelet growth factor and mediator lysophosphatidic acid (LPA) controls key regenerative responses of human gingival fibroblasts (GFs) and periodontal ligament fibroblasts (PDLFs) and positively modulates their responses to platelet-derived growth factor (PDGF). This study determined which LPA receptor (LPAR) subtype(s) LPA signals through to stimulate mitogenic extracellular signal-regulated kinase (ERK) 1/2 signaling and chemotaxis and to elicit intracellular Ca(2+) increases in GFs and PDLFs because many healing responses are calcium-dependent.

METHODS

Activation of mitogen-activated protein kinase was determined using Western blotting with an antibody to phosphorylated ERK1/2. Migration responses were measured using a microchemotaxis chamber. GF and PDLF intracellular Ca(2+) mobilization responses to multiple LPA species and LPAR subtype-specific agonists were measured by using a cell-permeable fluorescent Ca(2+) indicator dye.

RESULTS

LPA stimulated ERK1/2 phosphorylation via LPA(1)(-3). For GFs, LPA(1) preferentially elicited chemotaxis, and LPA(1-3) for PDLFs, as confirmed using subtype-specific agonists. Elevation of intracellular calcium seems to be mediated through LPA(1) and LPA(3), with little, if any, contribution from LPA(2).

CONCLUSIONS

To the best of our knowledge, this study provides the first evidence that LPA signals through specific LPAR subtypes to stimulate human oral fibroblast regenerative responses. These data, in conjunction with our previous findings showing that LPA modulates GF and PDLF responses to PDGF, suggest that LPA is a factor of emerging importance to oral wound healing.

Sp-1 and c-Myc mediate lysophosphatidic acid-induced expression of vascular endothelial growth factor in ovarian cancer cells via a hypoxia-inducible factor-1-independent mechanism

PURPOSE

Lysophosphatidic acid (LPA), which is present in ascites of ovarian cancer patients, stimulates expression of vascular endothelial growth factor (VEGF). VEGF is essential for the development and abdominal dissemination of ovarian cancer. We examined how LPA drives VEGF expression to gain a better understanding of tumor angiogenesis under normoxic conditions.

EXPERIMENTAL DESIGN

ELISA, Northern blotting, immunoblotting, quantitative PCR, and promoter reporter analysis in combination with small interfering RNA and pharmacologic inhibitors were used to examine LPA-induced VEGF expression and the underlying mechanisms.

RESULTS

LPA stimulated expression of multiple VEGF variants. A 123-bp fragment proximal to the transcriptional initiation site was identified to be functional promoter region responsible for the response to LPA. The fragment harbors consensus sites for several transcription factors including c-Myc and Sp-1 but not hypoxia-inducible factor-1. Blockade of Rho, ROCK, or c-Myc reduced LPA-dependent VEGF production and promoter activation, suggesting that the G12/13-Rho-ROCK-c-Myc cascade partially contributes to VEGF induction by LPA. More significantly, the multiple Sp-1 sites within the responsive region of the VEGF promoter were essential for LPA-mediated transcription. LPA induced Sp-1 phosphorylation and DNA-binding and transcriptional activities. The silencing of Sp-1 expression with small interfering RNA or inhibition of Sp-1 with pharmacologic inhibitors blocked VEGF production induced by LPA.

CONCLUSIONS

LPA stimulates hypoxia-inducible factor-1-independent VEGF expression to promote tumor angiogenesis through activation of the c-Myc and Sp-1 transcription factors.

Lysophosphatidic acid stimulates prostaglandin E2 production in cultured stromal endometrial cells through LPA1 receptor

Lysophosphatidic acid (LPA) has been shown to be a potent modulator of prostaglandin (PG) secretion during the luteal phase of the estrous cycle in the bovine endometrium in vivo. The aims of the present study were to determine the cell types of the bovine endometrium (epithelial or stromal cells) responsible for the secretion of PGs in response to LPA, the cellular, receptor, intracellular, and enzymatic mechanisms of LPA action. Cultured bovine epithelial and stromal cells were exposed to LPA (10(-5)-10(-9) M), tumor necrosis factor alpha (TNFalpha; 10 ng/mL) or oxytocin (OT; 10(-7) M) for 24 h. LPA treatment resulted in a dose-dependent increase of PGE(2) production in stromal cells, but not in epithelial cells. LPA did not influence PGF(2alpha) production in stromal or epithelial cells. To examine which type of LPA G-protein-coupled receptor (LP-GPCR; LPA1, LPA2, or LPA3) is responsible for LPA action, stromal cells were preincubated with three selected blockers of LPA receptors: NAEPA, DGPP, and Ki16425 for 0.5 h, and then stimulated with LPA. Only Ki16425 inhibited the stimulatory effect of LPA on PGE(2) production and cell proliferation in the stromal cells. LPA-induced intracellular calcium ion mobilization was also inhibited only by Ki16425. Finally, we examined whether LPA-induced PGE(2) synthesis in stromal cells is via the influence on mRNA expression for the enzymes responsible for PGE(2) synthesis-PGE(2) synthase (PGES) and PG-endoperoxide synthase 2 (PTGS2). We demonstrated that the stimulatory effect of LPA on PGE(2) production in stromal cells is via the stimulation of PTGS2 and PGES mRNA expression in the cells. The overall results indicate that LPA stimulates PGE(2) production, cell viability, and intracellular calcium ion mobilization in cultured stromal endometrial cells via Ki16425-sensitive LPA1 receptors. Moreover, LPA exerts a stimulatory effect on PGE(2) production in stromal cells via the induction of PTGS2 and PGES mRNA expression.

J. Lysophosphatidic acid as a potential target for treatment and molecular diagnosis of epithelial ovarian cancers

Az ováriumtumorok mortalitása a legmagasabb a nőgyógyászati tumorok között. Ez egyrészt a késői diagnózisnak, másrészt a hatásos terápia hiányának következménye. Az ováriumtumorok karcinogenezise és metasztázisképzése egy komplikált genetikai, molekuláris és biokémiai folyamatsor eredménye. A lizofoszfátsav (LPA) termelésének, receptorstátusának és szignáltranszdukciós útvonalának abnormalitása gyakran megtalálható az ováriumtumorokban, ami azt sejteti, hogy az LPA nagyon fontos szerepet játszik ennek a betegségnek a kialakulásában és patofiziológiájában. Így jogosan feltételezhetjük, hogy az LPA-szignálkaszkád számos célpontot szolgáltat a molekuláris kezelési módok kialakítására és jó példát mutat arra, hogyan lehet új diagnosztikus és terápiás módszereket kialakítani egyes betegségek ellen. Az LPA-t lebontó és termelő enzimcsaládoknak csak a közelmúltban történt felfedezése és a receptorspecifikus molekulák kifejlesztése új fejezetet nyithat e potenciálisan halálos betegség kezelésében. Ebben az összefoglaló tanulmányban ismertetjük, hogy a tumorsejtekben lévő LPA-t lebontó enzimek aktivitása csökkent, és ez hozzájárul a tumor progressziójához. Ugyanezen enzimek mesterségesen létrehozott, fokozott aktivitása csökkenti a tumorsejtek növekedését és elősegíti a fiziológiás viszonyok helyreállását. Bemutatjuk azokat az irodalmi adatokat, amelyek egyértelműen bizonyítják, hogy a lipidfoszfát-foszfatáz enzimek hatásukat a sejten kívüli LPA lebontásával érik el. Minthogy ez a lebontás extracellulárisan történik, ez megmagyarázza a „bystander-effect” előfordulását, amit szintén ismertetünk. Az LPA-lebontás és -termelés enzimjei, illetve az LPA-t kötő receptorok kitűnő célpontok új molekuláris terápia kidolgozására. A különböző LPA-izoformák és más lizofoszfolipidek szintváltozásainak korai detektálása segíthet a tumor korai diagnosztizálásában, illetve később a kezelés hatékonyságának követésében. A közelmúlt jelentős LPA-szignálkaszkáddal kapcsolatos kutatási eredményei azt sejtetik, hogy azok jelentős szerepet fognak játszani ennek a még mindig halálos betegségnek a kezelésében, de további kutatások szükségesek a részletek pontos megértéséhez.

Identification and characterization of a novel lysophosphatidic acid receptor, p2y5/LPA6

p2y5 is an orphan G protein-coupled receptor that is closely related to the fourth lysophosphatidic acid (LPA) receptor, LPA4. Here we report that p2y5 is a novel LPA receptor coupling to the G13-Rho signaling pathway. "LPA receptor-null" RH7777 and B103 cells exogenously expressing p2y5 showed [3H]LPA binding, LPA-induced [35S]guanosine 5'-3-O-(thio)triphosphate binding, Rho-dependent alternation of cellular morphology, and Gs/13 chimeric protein-mediated cAMP accumulation. LPA-induced contraction of human umbilical vein endothelial cells was suppressed by small interfering RNA knockdown of endogenously expressed p2y5. We also found that 2-acyl-LPA had higher activity to p2y5 than 1-acyl-LPA. A recent study has suggested that p2y5 is an LPA receptor essential for human hair growth. We confirmed that p2y5 is a functional LPA receptor and propose to designate this receptor LPA6.

Lysophosphatidic acid receptor 2 and Gi/Src pathway mediate cell motility through cyclooxygenase 2 expression in CAOV-3 ovarian cancer cells

Lysophosphatidic acid (LPA) is a bioactive phospholipids and involves in various cellular events, including tumor cell migration. In the present study, we investigated LPA receptor and its transactivation to EGFR for cyclooxygenase-2 (COX-2) expression and cell migration in CAOV-3 ovarian cancer cells. LPA induced COX-2 expression in a dose-dependent manner, and pretreatment of the cells with pharmacological inhibitors of Gi (pertussis toxin), Src (PP2), EGF receptor (EGFR) (AG1478), ERK (PD98059) significantly inhibited LPA- induced COX-2 expression. Consistent to these results, transfection of the cells with selective Src siRNA attenuated COX-2 expression by LPA. LPA stimulated CAOV-3 cell migration that was abrogated by pharmacological inhibitors and antibody of EP2. Higher expression of LPA2 mRNA was observed in CAOV-3 cells, and transfection of the cells with a selective LPA2 siRNA significantly inhibited LPA-induced activation of EGFR and ERK, as well as COX-2 expression. Importantly, LPA2 siRNA also blocked LPA-induced ovarian cancer cell migration. Collectively, our results clearly show the significance of LPA2 and Gi/Src pathway for LPA-induced COX-2 expression and cell migration that could be a promising drug target for ovarian cancer cell metastasis.

Unique ligand selectivity of the GPR92/LPA5 lysophosphatidate receptor indicates role in human platelet activation

Lysophosphatidic acid (LPA) is a ligand for LPA(1-3) of the endothelial differentiation gene family G-protein-coupled receptors, and LPA(4-8) is related to the purinergic family G-protein-coupled receptor. Because the structure-activity relationship (SAR) of GPR92/LPA(5) is limited and whether LPA is its preferred endogenous ligand has been questioned in the literature, in this study we applied a combination of computational and experimental site-directed mutagenesis of LPA(5) residues predicted to interact with the headgroup of LPA. Four residues involved in ligand recognition in LPA(5) were identified as follows: R2.60N mutant abolished receptor activation, whereas H4.64E, R6.62A, and R7.32A greatly reduced receptor activation. We also investigated the SAR of LPA(5) using LPA analogs and other non-lysophospholipid ligands. SAR revealed that the rank order of agonists is alkyl glycerol phosphate > LPA > farnesyl phosphates >> N-arachidonoylglycine. These results confirm LPA(5) to be a bona fide lysophospholipid receptor. We also evaluated several compounds with previously established selectivity for the endothelial differentiation gene receptors and found several that are LPA(5) agonists. A pharmacophore model of LPA(5) binding requirements was developed for in silico screening, which identified two non-lipid LPA(5) antagonists. Because LPA(5) transcripts are abundant in human platelets, we tested its antagonists on platelet activation and found that these non-lipid LPA(5) antagonists inhibit platelet activation. The present results suggest that selective inhibition of LPA(5) may provide a basis for future anti-thrombotic therapies.

Infrequent mutation of lysophosphatidic Acid receptor-1 gene in hamster pancreatic duct adenocarcinomas and established cell lines

To evaluate the involvement of lysophosphatidic acid receptor-1 (LPA1) gene alteration in pancreatic carcinogenesis, we investigated mutations in the LPA1 gene in hamster pancreatic duct adenocarcinomas (PDAs) and established cell lines. Female Syrian golden hamsters received 30 mg/kg of N-nitrosobis(2-oxopropyl)amine (BOP) followed by repeated exposure to an augmentation pressure regimen consisting of a choline-deficient diet combined with DL-ethionine and then L-methionine and a further administration of 20 mg/kg BOP. A total of 10 PDAs obtained 10 weeks after beginning the experiment and three cell lines established from subcutaneously transplantable PDAs in syngeneic hamsters were examined for mutations using reverse transcription-polymerase chain reaction-single strand conformation polymorphism (RT-PCR-SSCP) analysis. A mutation was detected in only one PDA (1/10, 10%) in the form of a GGA to GTA (Gly to Val) transversion at codon 355, and no mutations were detected in the three cell lines. These results suggest that the LPA1 gene mutation may play roles in a limited fraction of BOP-induced pancreatic duct carcinogenesis in hamsters.

Altered activity of lysophospholipase D, which produces bioactive lysophosphatidic acid and choline, in serum from women with pathological pregnancy

Altered lipid metabolism is associated with human abnormal pregnancy, such as pre-eclampsia and preterm labor, and potentially leads to fetus loss. A causative factor for the onset and progress of the systemic multifactorial syndromes associated with the pathological pregnancy is oxidized low-density lipoprotein, an active identity of which was postulated to be lysophosphatidic acid (LPA). We previously found that LPA is produced extracellularly by plasma lysophospholipase D (lysoPLD) activity of autotaxin, a tumor cell motility-stimulating protein. In this study, a convenient assay based on the choline released from endogenous substrate or exogenous lysophosphatidylcholine (LPC) was used for comparison of serum lysoPLD activity among patients with normal and abnormal pregnancy. The serum choline-producing activity was found to be mainly due to autotaxin, and dependent on its dilution rate. There was some association between low dilution dependency of serum lysoPLD activity toward an exogenous LPC and high lysoPLD activity toward endogenous substrates in cases of patients with preterm labor and pre-eclampsia. However, there was no difference in the serum level of LPC between women with normal pregnancy and those with pathological pregnancy. These results indicate that production of bioactive LPA by lysoPLD activity is elevated by an unknown mechanism that may be related to increased availability of endogenous substrates LPC, but not its concentration in human serum. If the level of LPA in blood circulation is elevated in the pathological pregnancies in vivo, it may play a role in induction and/or progression of systemic vascular dysfunction seen patients with preterm labor or pre-eclampsia.

Gene expression profiles of lysophosphatidic acid-related molecules in the prostate: relevance to prostate cancer and benign hyperplasia

OBJECTIVE

To elucidate gene expression profiles of lysophosphatidic acid (LPA)-related molecules in cancer, pre-cancerous lesion, and benign hyperplasia of the prostate.

MATERIALS AND METHODS

Prostate tissue samples were surgically obtained from 10 patients with localized prostate cancer and seven patients with invasive bladder cancer. Cancer cells and the corresponding stromal cells from normal prostate, high grade intraepithelial neoplasia (HGPIN), benign hyperplastic glands were isolated by laser capture microdissection. mRNA levels of three LPA receptors, LPA1, LPA2, LPA3, two LPA-synthesizing enzymes, autotaxin (ATX), acylglycerol kinase (AGK), and a LPA-degradation enzyme, prostatic acid phosphatase (PAP), were quantitatively assessed. The expression levels of the same genes were also determined in three human prostate cancer cell lines LNCaP, PC-3, and DU-145.

RESULTS

LPA1 mRNA level was significantly decreased in HGPIN and cancer epithelia when compared to the benign glands. LPA3 mRNA level was elevated in cancer epithelia compared to benign glands. LPA3, AGK, and PAP were predominantly expressed in LNCaP cells while LPA1 and ATX gene expressions were found in PC-3 and Du-145 cells. In BPH, AGK was abundantly expressed in the stroma while PAP was predominant in epithelial cells.

CONCLUSIONS

By acting via LPA3, LPA may play an important role in the development of prostate cancer. Switching of LPA receptor expression from LPA3 to LPA1, may be involved in prostate cancer progression and/or androgen independence. LPA may also play a key role in the development of benign prostatic hyperplasia.

Modulation of epidermal growth factor receptor binding to human airway smooth muscle cells by glucocorticoids and beta2-adrenergic receptor agonists

EGF receptors (EGFRs) are increased in airway smooth muscle in asthma, which may contribute to both their hyperproliferation and hypercontractility. Lysophosphatidic acid (LPA) is a candidate pathological agent in asthma and other airway diseases, and LPA upregulates EGFRs in human airway smooth muscle (HASM) cells. We tested whether therapeutic glucocorticoids and/or beta(2)-adrenergic receptor (beta(2)AR) agonists also alter EGFR binding in HASM cells. Exposure to glucocorticoids for 24 h induced a twofold increase in EGFR binding similar to that with LPA; fluticasone was markedly more potent than dexamethasone. The increase in EGFR binding by glucocorticoids required 24-h exposure, consistent with transcription-mediated effects. Although the increase in EGFR binding was blocked by the protein synthesis inhibitor cycloheximide for LPA, fluticasone, and dexamethasone, only LPA induced a significant increase in EGFR protein expression detected by immunoblotting. In contrast to the increased binding induced by the glucocorticoids, the beta(2)AR agonists isoproterenol, albuterol, and salmeterol all induced a decrease in EGFR binding. beta(2)AR agonist effects were multiphasic, with an initial decline at 2-4 h that reversed by 6 h and a second, somewhat greater decrease by 18-24 h. In cells pretreated with glucocorticoids, the decreases in EGFR binding by subsequent beta(2)AR treatment were not statistically significant; glucocorticoid upregulation of EGFRs also prevented further increases by LPA. Similar increases by glucocorticoids and decreases by beta(2)AR agonists were found in HFL-1 human lung fibroblasts. These complex and opposing effects of clinically relevant glucocorticoids and beta(2)AR agonists on airway mesenchymal cell EGFRs likely contribute to their overall therapeutic profile in the diseased airway.

Qualitative and quantitative analyses of phospholipids by LC-MS for lipidomics

In this chapter we are going to mention about three different approaches in lipidomics and how to effectively profile or calculate the amounts of phospholipids from major molecular species up to minor ones. 1) Precise identification and profiling of individual molecular species of phospholipids by data-dependent LC-ESIMS/MS combination with "Lipid Search". We have been using this method as a global analysis of phospholipid. We usually applied this method at least once for new biological samples. We constructed an automated search engine, "Lipid Search", for identification and profiling of phospholipids. Once after applying this analysis, a specified retention time can be obtained for each elution peak of individual phospholipid molecular species. Thus, reproducible identification results can be effectively obtained by our search engine from the data obtained by single LC or combination of LC with specified head group survey by using precursor ion scanning or neutral loss scanning. 2) An effective analytical method of LC-ESIMS for the identification of acidic phospholipids such as phosphatidic acid and phosphatidylserine. This is an approach of how to obtain sharp chromatographic peaks for acidic lipids such as phosphatidic acid and phosphatidylserine that are normally detected as broad elution peaks. With this improvement very small amount of molecular species in minor acidic phospholipids were effectively obtained. 3) Identification and profiling of molecular species in focused phospholipids. Third one is a combination analysis of focused methods such as precursor ion scanning or neutral loss scanning and high efficient LC separation. As reported previously, different combinations of fatty acids on sn-1 and sn-2 can be mostly detected as separate peaks by reverse phase LC-ESIMS. Detection limit of precursor ion scanning or neutral loss scanning is more than ten times higher than that of the method without LC separation, because of decreased ion suppression. We will mention about application of this methods for focused analysis on phosphatidylethanolamine-plasmalogens.

Lipid metabolites as regulators of airway smooth muscle function

Compelling evidence identifies airway smooth muscle (ASM) not only as a target but also a cellular source for a diverse range of mediators underlying the processes of airway narrowing and airway hyperresponsiveness in diseases such as asthma. These include the growing family of plasma membrane phospholipid-derived polyunsaturated fatty acids broadly characterised by the prostaglandins, leukotrienes, lipoxins, isoprostanes and lysophospholipids. In this review, we describe the enzymatic and non-enzymatic biosynthetic pathways of these lipid mediators and how these are influenced by drug treatment, oxidative stress and airways disease. Additionally, we outline their cognate receptors, many of which are expressed by ASM. We describe potential deleterious and protective roles for these lipid mediators in airway inflammatory and remodelling processes by describing their effects on diverse functions of ASM in asthma that have the potential to contribute to asthma pathogenesis and symptoms. These functions include contractile tone development, cytokine and extracellular matrix production, and cellular proliferation and migration.

Lysophosphatidylserine regulates blood glucose by enhancing glucose transport in myotubes and adipocytes

Lysophosphatidylserine (LPS) is known to have diverse cellular effects, but although LPS is present in many biological fluids, its in vivo effects have not been elucidated. In the present study, we investigated the effects of LPS on glucose metabolism in vivo, and how skeletal muscle cells respond to LPS stimulation. LPS enhanced glucose uptake in a dose- and time-dependent manner in L6 GLUT4myc myotubes, and this effect of LPS on glucose uptake was mediated by a Galpha(i) and PI 3-kinase dependent signal pathway. LPS increased the level of GLUT4 on the cell surface of L6 GLUT4myc myotubes, and enhanced glucose uptake in 3T3-L1 adipocytes. In line with its cellular functions, LPS lowered blood glucose levels in normal mice, while leaving insulin secretion unaffected. LPS also had a glucose-lowering effect in STZ-treated type 1 diabetic mice and in obese db/db type 2 diabetic mice. This study shows that LPS-stimulated glucose transport both in skeletal muscle cells and adipocytes, and significantly lowered blood glucose levels both in type 1 and 2 diabetic mice. Our results suggest that LPS is involved in the regulation of glucose homeostasis in skeletal muscle and adipose tissue.

Mutations of lysophosphatidic acid receptor-1 gene during progression of lung tumors in rats

Lysophosphatidic acid (LPA) is a bioactive phospholipid that stimulates cell proliferation, migration, and protects cells from apoptosis. It interacts with specific G protein-coupled transmembrane receptors. In this study, mutations of lysophosphatidic acid receptor-1 (LPA1) gene were investigated to clarify the possible molecular mechanisms underlying the development of lung tumors induced by N-nitrosobis(2-hydroxypropyl)amine (BHP) in rats. Male Wistar rats, 6 weeks of age, were given 2000ppm BHP in their drinking water for 12 weeks and then maintained without further treatment until sacrifice at 25 weeks. Genomic DNAs were extracted from paraffin-embedded tissues and exons 2-4 were examined for mutations, using polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) analysis. No LPA1 mutations were detected in 15 hyperplasias, but 2 out of 12 adenomas (16.7%) and 7 out of 17 adenocarcinomas (41.2%). These results suggest that mutations of LPA1 gene may be involved in the acquisition of growth advantage from adenomas to adenocarcinomas in lung carcinogenesis induced in rats by BHP.

Lysophosphatidic acid receptors determine tumorigenicity and aggressiveness of ovarian cancer cells

BACKGROUND

Lysophosphatidic acid (LPA) acts through the cell surface G protein-coupled receptors, LPA1, LPA2, or LPA3, to elicit a wide range of cellular responses. It is present at high levels in intraperitoneal effusions of human ovarian cancer increasing cell survival, proliferation, and motility as well as stimulating production of neovascularizing factors. LPA2 and LPA3 and enzymes regulating the production and degradation of LPA are aberrantly expressed by ovarian cancer cells, but the consequences of these expression changes in ovarian cancer cells were unknown.

METHODS

Expression of LPA1, LPA2, or LPA3 was inhibited or increased in ovarian cancer cells using small interfering RNAs (siRNAs) and lentivirus constructs, respectively. We measured the effects of changes in LPA receptor expression on cell proliferation (by crystal violet staining), cell motility and invasion (using Boyden chambers), and cytokines (interleukin 6 [IL-6], interleukin 8 [IL-8], and vascular endothelial growth factor [VEGF]) production by enzyme-linked immunosorbent assay. The role of LPA receptors in tumor growth, ascites formation, and cytokine production was assessed in a mouse xenograft model. All statistical tests were two-sided.

RESULTS

SKOV-3 cells with increased expression of LPA receptors showed increased invasiveness, whereas siRNA knockdown inhibited both migration (P < .001, Student t test) and invasion. Knockdown of the LPA2 or LPA3 receptors inhibited the production of IL-6, IL-8, and VEGF in SKOV-3 and OVCAR-3 cells. SKOV-3 xenografts expressing LPA receptors formed primary tumors of increased size and increased ascites volume. Invasive tumors in the peritoneal cavity occurred in 75% (n = 4) of mice injected with LPA1 expressing SKOV-3 and 80% (n = 5) of mice injected with LPA2 or LPA3 expressing SKOV-3 cells. Metastatic tumors expressing LPA1, LPA2, and LPA3 were identified in the liver, kidney, and pancreas; tumors expressing LPA2 and LPA3 were detected in skeletal muscle; and tumors expressing LPA2 were also found in the cervical lymph node and heart. The percent survival of mice with tumors expressing LPA2 or LPA3 was reduced in comparison with animals with tumors expressing beta-galactosidase.

CONCLUSIONS

Expression of LPA2 or LPA3 during ovarian carcinogenesis contributes to ovarian cancer aggressiveness, suggesting that the targeting of LPA production and action may have potential for the treatment of ovarian cancer.

Frequent mutations of lysophosphatidic acid receptor-1 gene in rat liver tumors

Lysophosphatidic acid (LPA) is a bioactive phospholipid that stimulates cell proliferation, migration, and protects cells from apoptosis. It interacts with specific G protein-coupled transmembrane receptors, including LPA1 to LPA5. In the present study, to clarify an involvement of LPA1 gene alterations in the development of hepatocellular carcinomas (HCCs) we investigated the LPA1 mutations in rat HCCs induced by exogenous and endogenous liver carcinogenesis models. We induced HCCs in rats with N-nitrosodiethylamine (DEN) and a choline-deficient l-amino acid-defined (CDAA) diet. RNAs were extracted from 15 HCCs induced by DEN and 12 HCCs induced by the CDAA diet. To identify LPA1 mutations, reverse transcription (RT) - polymerase chain reaction (PCR) - single strand conformation polymorphism (SSCP) analysis, followed by nucleotide sequencing, was performed. Missense mutations were detected in 7 out of 15 HCCs (46.7%) induced by DEN. Five out of 12 HCCs (41.7%) induced by the CDAA diet also showed missense mutations. These results demonstrated that mutations in LPA1 gene occur in rat HCCs induced by DEN and the CDAA diet, suggesting that LPA1 mutations may be essentially involved in rat liver carcinogenesis.

Role of LPA4/p2y9/GPR23 in negative regulation of cell motility

Lysophosphatidic acid (LPA) is a ligand of multiple G protein-coupled receptors. The LPA(1-3) receptors are members of the endothelial cell differentiation gene (Edg) family. LPA(4)/p2y9/GPR23, a member of the purinergic receptor family, and recently identified LPA(5)/GPR92 and p2y5 are structurally distant from the canonical Edg LPA receptors. Here we report targeted disruption of lpa(4) in mice. Although LPA(4)-deficient mice displayed no apparent abnormalities, LPA(4)-deficient mouse embryonic fibroblasts (MEFs) were hypersensitive to LPA-induced cell migration. Consistent with negative modulation of the phosphatidylinositol 3 kinase pathway by LPA(4), LPA(4) deficiency potentiated Akt and Rac but decreased Rho activation induced by LPA. Reconstitution of LPA(4) converted LPA(4)-negative cells into a less motile phenotype. In support of the biological relevance of these observations, ectopic expression of LPA(4) strongly inhibited migration and invasion of human cancer cells. When coexpressed with LPA(1) in B103 neuroblastoma cells devoid of endogenous LPA receptors, LPA(4) attenuated LPA(1)-driven migration and invasion, indicating functional antagonism between the two subtypes of LPA receptors. These results provide genetic and biochemical evidence that LPA(4) is a suppressor of LPA-dependent cell migration and invasion in contrast to the motility-stimulating Edg LPA receptors.

Lysophosphatic acid modulates prostaglandin secretion in the bovine uterus

Lysophosphatidic acid (LPA) modulates prostaglandin (PG) synthesis via LPA receptor 3 (LPAR3) in the murine endometrium. The lack of functional LPAR3 in mice may lead to embryo mortality. In the present study, we examined the role of LPA in the bovine uterus. We confirmed that LPA is locally produced and released from the bovine endometrium. Moreover, there are enzymes involved in LPA synthesis (phospholipase (PL) D(2) and PLA2G1B) in the bovine endometrium during estrous cycle and early pregnancy. Expression of the receptor for LPA (LPAR1) was positively correlated with the expression of PGE(2) synthase (PGES) and negatively correlated with the expression of PGF(2alpha) synthase (aldose reductase with 20 alpha-hydroxysteroid dehydrogenase activity - PGFS) during early pregnancy. In vivo LPA induced P4 and PGE(2) secretion was inhibited by LPAR1 antagonist (Ki16425). The overall results indicate that LPA is locally produced and released from the bovine endometrium. Moreover, LPAR1 gene expression in the endometrium during the estrous cycle and early pregnancy indicates that LPA may play autocrine and/or paracrine roles in the bovine uterus. LPAR1 gene expression is positively correlated with the expression of the enzyme responsible for luteotropic PGE(2) production (PGES) in endometrium. In cow, LPA stimulates P4 and PGE(2) secretion. Thus, LPA in the bovine reproductive tract may indirectly (via endometrium) or directly support corpus luteum action via the increase of P4 synthesis and the increase of PGE(2)/PGF(2)(alpha) ratio. It suggests that LPA may serve as an important factor in the maintenance of early pregnancy in cow.

Inhibition of lysophosphatidic acid receptor-2 expression by RNA interference decreases lysophosphatidic acid-induced urokinase plasminogen activator activation, cell invasion, and migration in ovarian cancer SKOV-3 cells

AIM

To explore the role of lysophosphatidic acid receptor-2 (LPA2) in regulating lysophosphatidic acid (LPA)-induced urokinase plasminogen activator (uPA) activation, cell invasion, and migration in human ovarian cancer cell line SKOV-3.

METHODS

SKOV-3 cells were stimulated with LPA. Cell supernatant uPA level and activity were measured using enzyme-linked immunosorbent assay. LPA2 mRNA expression was inhibited with LPA2-specific small interfering RNA (siRNA) and examined using semiquantitative reverse transcriptase-polymerase chain reaction. LPA-induced cell invasion and migration in transfected cells were evaluated by a Matrigel invasion chamber and a Transwell chemotaxis chamber, respectively.

RESULTS

LPA stimulation significantly enhanced in vitro uPA activity in time- and dose-dependent manner. The levels of LPA-induced uPA protein decreased by 55% in LPA2 siRNA-transfected cells compared with negatively transfected cells at 24 hours after being treated with 80 micromol/L LPA (0.75+/-0.03 vs 0.34+/-0.04, P=0.004). In the LPA2 specific siRNA-transfected SKOV-3 cells, LPA treatment at 80 micromol/L induced considerably less invasion and migration compared with negative control siRNA-transfected SKOV-3 cells (invasion: 178+/-17.2 vs 36.2+/-3.3, P=0.009; migration: 220.4+/-25.5 vs 57+/-7.6, P=0.009).

CONCLUSION

LPA2 has an essential role in LPA-induced uPA activation and tumor cell invasion in ovarian cancer SKOV-3 cells.

Biological roles of lysophospholipid receptors revealed by genetic null mice: an update

Two lysophospholipids (LPs), lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), are known to affect various cellular events. Their actions are mediated by binding to at least ten bona fide high-affinity G protein-coupled receptors referred to as LPA1-5 and S1P1-5. These LPs are expressed throughout the body and are involved in a range of biological activities including normal development, as well as functioning in most organ systems. A growing number of biological functions have been uncovered in vivo using single- or multiple-null mice for each LP receptor. This review will focus on findings from in vivo as well as in vitro studies using genetic null mice for the LP receptors, LPA1,2,3 and S1P1,2,3,5, and for the LP producing enzymes, autotaxin and sphingosine kinase 1/2.

Phosphatase-resistant analogues of lysophosphatidic acid: agonists promote healing, antagonists and autotaxin inhibitors treat cancer

Isoform-selective agonists and antagonists of the lysophosphatidic acid (LPA) G protein-coupled receptors (GPCRs) have important potential applications in cell biology and therapy. LPA GPCRs regulate cancer cell proliferation, invasion, angiogenesis, and also biochemical resistance to chemotherapy- and radiotherapy-induced apoptosis. LPA and its analogues also are feedback inhibitors of the enzyme lysophospholipase D (lysoPLD, a.k.a., autotaxin, ATX), a central regulator of invasion and metastasis. For cancer therapy, the optimal therapeutic profile would be a metabolically-stabilized, pan-LPA receptor antagonist that also inhibited lysoPLD. For protection of gastrointestinal mucosa and lymphocytes, LPA agonists would be desirable to minimize or reverse radiation or chemical-induced injury. Analogues of lysophosphatidic acid (LPA) that are chemically modified to be less susceptible to phospholipases and phosphatases show activity as long-lived receptor-specific agonists and antagonists for LPA receptors, as well as inhibitors for the lysoPLD activity of ATX.

Serum autotaxin measurement in haematological malignancies: a promising marker for follicular lymphoma

Autotaxin (ATX) is a tumour cell motility-stimulating factor originally isolated from melanoma cell supernatants. ATX is identical to lysophospholipase D, which produces a bioactive lipid mediator, lysophosphatidic acid (LPA), from lysophosphatidylcholine. ATX is overexpressed in various malignancies, including Hodgkin lymphoma, and ATX may stimulate tumour progression via LPA production. The present study measured the serum ATX antigen levels in patients with haematological malignancies using a recently developed automated enzyme immunoassay. The serum ATX antigen levels in patients with B-cell neoplasms, especially follicular lymphoma (FL), were higher than those in healthy subjects. Serum ATX antigen levels in FL patients were associated with tumour burden and changed in parallel with the patients' clinical courses. The serum ATX antigen levels were little affected by inflammation, unlike the soluble interleukin-2 receptor and beta2-microglobulin levels. As expected, the plasma LPA levels in FL patients were correlated with the serum ATX antigen levels. Given that leukaemic tumour cells from FL patients expressed ATX, the shedding of ATX from lymphoma cells probably leads to the elevation of serum ATX antigen levels. Our results suggest that the serum ATX antigen level may be a promising and novel marker for FL.

Alkoxymethylenephosphonate analogues of (Lyso) phosphatidic acid stimulate signaling networks coupled to the LPA2 receptor

An efficient stereocontrolled synthesis afforded alkoxymethylenephosphonate (MP) analogues of lysophosphatidic acid (LPA) and phosphatidic acid (PA). The pharmacological properties of MP-LPA and MP-PA analogues were characterized for LPA receptor subtype-specific agonist and antagonist activity using Ca(2+)-mobilization assays in RH7777 cells expressing the individual LPA(1)-LPA(3) receptors and CHO cells expressing LPA(4). In addition, activation of a PPARgamma reporter gene construct expressed in CV-1 cells was assessed. These metabolically stabilized LPA analogues exhibited an unexpected pattern of partial agonist/antagonist activity for the LPA G-protein-coupled receptor family and the intracellular LPA receptor PPARgamma. Analogues were compared with 18:1 LPA for activation of downstream signaling in HT-29 colon cancer cells, which exclusively express LPA(2), and both SKOV3 and OVCAR3 ovarian cancer cells, which express LPA(1), LPA(2), and LPA(3). Unexpectedly, reverse phase protein arrays showed that four MP-LPA and MP-PA analogues selectively activated downstream signaling in HT-29 cells with greater potency than LPA. In particular, the oleoyl MP-LPA analogue strongly promoted phosphorylation and activation of AKT, MEK, and pS6 in HT-29 cells in a concentration-dependent manner. In contrast, the four MP-LPA and MP-PA analogues were equipotent with LPA for pathway activation in the SKOV3 and OVCAR3 cells. Taken together, these results suggest that the MP analogues may selectively activate signaling via the LPA(2) receptor subtype, while simultaneously suppressing signaling through the LPA(1) and LPA(3) subtypes.

Lysophosphatidic acid stimulates the proliferation and motility of malignant pleural mesothelioma cells through lysophosphatidic acid receptors, LPA1 and LPA2

Lysophosphatidic acid (LPA) is one of the simplest natural phospholipids. This phospholipid is recognized as an extracellular potent lipid mediator with diverse effects on various cells. Although LPA is shown to stimulate proliferation and motility via LPA receptors, LPA(1) and LPA(2), in several cancer cell lines, the role of LPA and LPA receptors for malignant pleural mesothelioma (MPM) has been unknown. MPM is an aggressive malignancy with a poor prognosis and the incidence is increasing and is expected to increase further for another 10-20 years worldwide. Therefore, the development of novel effective therapies is needed urgently. In this study, we investigated the effect of LPA on the proliferation and motility of MPM cells. We found that all 12 cell lines and four clinical samples of MPM expressed LPA(1), and some of them expressed LPA(2), LPA(3), LPA(4) and LPA(5). LPA stimulated the proliferation and motility of MPM cells in a dose-dependent manner. Moreover, LPA-induced proliferation was inhibited by Ki16425, an inhibitor of LPA(1), and small interfering RNA against LPA(1), but not LPA(2). Interestingly, LPA-induced motility was inhibited by small interfering RNA against LPA(2), but not LPA(1), unlike a number of previous reports. These results indicate that LPA is a critical factor on proliferation though LPA(1), and on motility though LPA(2) in MPM cells. Therefore, LPA and LPA receptors, LPA(2) as well as LPA(1), represent potential therapeutic targets for patients with MPM.

Lysophosphatidic acid-induced membrane ruffling and brain-derived neurotrophic factor gene expression are mediated by ATP release in primary microglia

We examined the effects of lysophosphatidic acid (LPA) on microglia, which may play an important role in the development and maintenance of neuropathic pain. LPA caused membrane ruffling as detected by scanning electron microscopy, and increased the expression of brain-derived neurotrophic factor (BDNF) in a primary culture of rat microglia, which express LPA(3), but not LPA(1) or LPA(2) receptors. These actions were inhibited by a Galpha(q/11)-antisense oligodeoxynucleotide (AS-ODN), U73122, an inhibitor of phospholipase C (PLC), and apyrase, which specifically degrades ATP and ADP. When ATP release was measured using a luciferin-luciferase bioluminescence assay, LPA was shown to increase it in an LPA(3) and PLC inhibitor-reversible manner. However, LPA-induced ATP release was also blocked by the Galpha(q/11) AS-ODN, but not by pertussis toxin. These results suggest that LPA induces the release of ATP from rat primary cultured microglia via the LPA(3) receptor, Galpha(q/11) and PLC, and that the released ATP or ectopically converted ADP may in turn cause membrane ruffling via P2Y(12) receptors and Galpha(i/o) activation, and BDNF expression via activation of P2X(4) receptors.

ERK1/2 mediate wounding- and G-protein-coupled receptor ligands-induced EGFR activation via regulating ADAM17 and HB-EGF shedding

PURPOSE

Previous studies have shown that wounding of human corneal epithelial cells (HCECs) results in the release of G-protein-coupled receptor ligands such as ATP and lysophosphatidic acid (LPA), which in turn transactivate epidermal growth factor (EGF) receptor (EGFR) through ectodomain shedding of heparin-binding EGF-like growth factor (HB-EGF). In the present study, the role of extracellular signal-regulated kinases 1/2 (ERK1/2) in regulating EGFR transactivation was investigated.

METHODS

SV40-immortalized HCECs were wounded or stimulated with ATP and LPA. EGFR and ADAM17 activation was analyzed by immunoprecipitation followed by Western blot analysis with phospho-tyrosine or phospho-serine antibodies, respectively. Phosphorylation of ERK and AKT was analyzed by Western blot analysis. HB-EGF shedding was assessed by measuring the release of alkaline phosphatase (AP) in a stably transfected human corneal epithelial (THCE) cell line expressing HB-EGF-AP. ADAM17 and ERK interaction was determined by coimmunoprecipitation.

RESULTS

Early, but not late, ERK1/2 phosphorylation in response to wounding, LPA, and ATP was EGFR independent, but sensitive to the inhibitors of calcium influx, protein kinase C and Src kinase. Wounding-, LPA-, and ATP-induced HB-EGF shedding and EGFR activation were attenuated by the MAPK/ERK kinase (MEK) inhibitors PD98059 and U0126, as well as by ADAM10 and -17 inhibitors. ADAM17 was found to be physically associated with active ERK and phosphorylated at serine residues in an ERK-dependent manner in wounded cells.

CONCLUSIONS

Taken together, our data suggest that in addition to functioning as an EGFR downstream effector, ERK1/2 also mediates ADAM-dependent HB-EGF shedding and subsequent EGFR transactivation in response to a variety of stimuli, including wounding and GPCR ligands.

Lysophospholipid signaling in the function and pathology of the reproductive system

BACKGROUND

Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two prominent signaling lysophospholipids (LPs) exerting their functions through a group of G protein-coupled receptors (GPCRs). This review covers current knowledge of the LP signaling in the function and pathology of the reproductive system.

METHODS

PubMed was searched up to May 2008 for papers on lysophospholipids/LPA/S1P/LPC/SPC in combination with each part of the reproductive system, such as testis/ovary/uterus.

RESULTS

LPA and SIP are found in significant amounts in serum and other biological fluids. To date, 10 LP receptors have been identified, including LPA(1-5) and S1P(1-5). In vitro and in vivo studies from the past three decades have demonstrated or suggested the physiological functions of LP signaling in reproduction, such as spermatogenesis, male sexual function, ovarian function, fertilization, early embryo development, embryo spacing, implantation, decidualization, pregnancy maintenance and parturition, as well as pathological roles in ovary, cervix, mammary gland and prostate cancers.

CONCLUSIONS

Receptor knock-out and other studies indicate tissue-specific and receptor-specific functions of LP signaling in reproduction. More comprehensive studies are required to define mechanisms of LP signaling and explore the potential use as a therapeutic target.

Identification of non-lipid LPA3 antagonists by virtual screening

In the present study, we utilized virtual screening to identify LPA(3) antagonists. We have developed a three-point structure-based pharmacophore model based on known LPA(3) antagonists. This model was used to mine the NCI database. Docking, pharmacophore development, and database mining produced new, non-lipid leads. Experimental testing of seven computationally selected pharmacophore hits produced one potentiator and three antagonists, one of which displays both LPA(3) selectivity and nanomolar potency. Similarity searching in the ChemBridge database using the most promising lead as the search target produced four additional LPA(3) antagonists and a potent dual LPA(1&2) antagonist.

Lysophosphatidic acid induces rapid and sustained decreases in epidermal growth factor receptor binding via different signaling pathways in BEAS-2B airway epithelial cells

Lysophosphatidic acid (LPA) and epidermal growth factor (EGF) are important mediators of lung cell function and lung diseases. We showed previously that LPA decreases epidermal growth factor receptor (EGFR) binding rapidly in BEAS-2B airway epithelial cells, and this decrease is sustained to at least 18 h. The current studies investigate which LPA signaling pathways mediate the rapid versus sustained decreases in EGFR binding in BEAS-2B cells. The G(i/o) inhibitor pertussis toxin and the Rho kinase inhibitor Y-27632 [(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide] had no effect on the rapid or sustained decreases. However, the mitogen-activated protein kinase kinase (MEK) inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)-butadiene ethanolate] decreased extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, completely inhibited the rapid decrease in binding, and partially inhibited the sustained decrease. The direct Ca2+- and phospholipid-dependent protein kinase (PKC) activator phorbol-12-myristate-13-acetate stimulated ERK1/2 phosphorylation and decreased EGFR binding at both 15 min and 18 h. Furthermore, inhibitors of PKC partially inhibited ERK1/2 phosphorylation and the 15-min decrease but completely inhibited the 18-h decrease. Inhibitor time course studies showed that PKC induction of the 18-h decrease occurred during the first 3 h of treatment. We showed previously that LPA-stimulated EGFR transactivation contributes to the rapid decrease. Two transactivation inhibitors partially inhibited ERK1/2 phosphorylation, and U0126 partially inhibited EGFR transactivation, indicating that MEK may be involved both upstream and downstream of EGFR activation. Together, the data presented here indicate that LPA mediates the rapid decrease in EGFR binding via EGFR transactivation, MEK/ERK, and PKC, whereas the sustained decrease is regulated primarily by PKC.

G(q)-dependent signalling by the lysophosphatidic acid receptor LPA(3) in gastric smooth muscle: reciprocal regulation of MYPT1 phosphorylation by Rho kinase and cAMP-independent PKA

The present study characterized the signalling pathways initiated by the bioactive lipid, LPA (lysophosphatidic acid) in smooth muscle. Expression of LPA(3) receptors, but not LPA(1) and LPA(2), receptors was demonstrated by Western blot analysis. LPA stimulated phosphoinositide hydrolysis, PKC (protein kinase C) and Rho kinase (Rho-associated kinase) activities: stimulation of all three enzymes was inhibited by expression of the G(alphaq), but not the G(alphai), minigene. Initial contraction and MLC(20) (20 kDa regulatory light chain of myosin II) phosphorylation induced by LPA were abolished by inhibitors of PLC (phospholipase C)-beta (U73122) or MLCK (myosin light-chain kinase; ML-9), but were not affected by inhibitors of PKC (bisindolylmaleimide) or Rho kinase (Y27632). In contrast, sustained contraction, and phosphorylation of MLC(20) and CPI-17 (PKC-potentiated inhibitor 17 kDa protein) induced by LPA were abolished selectively by bisindolylmaleimide. LPA-induced activation of IKK2 {IkappaB [inhibitor of NF-kappaB (nuclear factor kappaB)] kinase 2} and PKA (protein kinase A; cAMP-dependent protein kinase), and degradation of IkappaBalpha were blocked by the RhoA inhibitor (C3 exoenzyme) and in cells expressing dominant-negative mutants of IKK2(K44A) or RhoA(N19RhoA). Phosphorylation by Rho kinase of MYPT1 (myosin phosphatase targeting subunit 1) at Thr(696) was masked by phosphorylation of MYPT1 at Ser(695) by PKA derived from IkappaB degradation via RhoA, but unmasked in the presence of PKI (PKA inhibitor) or C3 exoenzyme and in cells expressing IKK2(K44A). We conclude that LPA induces initial contraction which involves activation of PLC-beta and MLCK and phosphorylation of MLC(20), and sustained contraction which involves activation of PKC and phosphorylation of CPI-17 and MLC(20). Although Rho kinase was activated, phosphorylation of MYPT1 at Thr(696) by Rho kinase was masked by phosphorylation of MYPT1 at Ser(695) via cAMP-independent PKA derived from the NF-kappaB pathway.

Volume-sensitive NADPH oxidase activity and taurine efflux in NIH3T3 mouse fibroblasts

Reactive oxygen species (ROS) are produced in NIH3T3 fibroblasts during hypotonic stress, and H(2)O(2) potentiates the concomitant release of the organic osmolyte taurine (Lambert IH. J Membr Biol 192: 19-32, 2003). The increase in ROS production [5-(and-6)-carboxy-2', 7'-dichlorodihydrofluorescein diacetate fluorescence] is detectable after a reduction in the extracellular osmolarity from 335 mosM (isotonic) to 300 mosM and reaches a maximal value after a reduction to 260 mosM. The swelling-induced ROS production is reduced by the flavoprotein inhibitor diphenylene iodonium chloride (25 microM) but is unaffected by the nitric oxide synthase inhibitor N omega-nitro-l-arginine methyl ester, indicating that the volume-sensitive ROS production is NADPH oxidase dependent. NIH3T3 cells express the NADPH oxidase components: p22 phox, a NOX4 isotype; p47 phox; and p67 phox (real-time PCR). Exposure to the Ca2+-mobilizing agonist ATP (10 microM) potentiates the release of taurine but has no effect on ROS production under hypotonic conditions. On the other hand, addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA, 100 nM) or the lipid messenger lysophosphatidic acid (LPA, 10 nM) potentiates the swelling-induced taurine release as well as the ROS production. Overexpression of Rac1 or p47 phox or p47 phox knockdown [small interfering (si)RNA] had no effect on the swelling-induced ROS production or taurine release. NOX4 knockdown (siRNA) impairs the increase in the ROS production and the concomitant taurine release following osmotic exposure. It is suggested that a NOX4 isotype plus p22 phox account for the swelling-induced increase in the ROS production in NIH3T3 cells and that the oxidase activity is potentiated by PKC and LPA but not by Ca2+.

Lysophosphatidylserine induces calcium signaling through Ki16425/VPC32183-sensitive GPCR in bone marrow-derived mast cells and in C6 glioma and colon cancer cells

Lysophosphatidylserine (LPS) can be generated following phosphatidylserine-specific phospholipase A2 activation. The effects of LPS on cellular activities and the identities of its target molecules, however, have not been fully elucidated. In this study, we observed that LPS stimulated intracellular calcium increased in mouse bone marrow-derived mast cells (BMMC), and rat C6 glioma and human HCT116 colon cancer cells and compared the LPS-induced Ca2+ increases with the response by lysophosphatidic acid (LPA), a structurally related bioactive lysolipid. In order to test involvement of signaling molecules in the LPS-induced Ca2+ signaling, we used pertussis toxin (PTX), U73122, and 2-APB, which are specific inhibitors for G proteins, phospholipase C (PLC), and IP3 receptors, respectively. The increases due to LPS and LPA were inhibited by PTX, U-73122 and 2-APB, suggesting that both lipids stimulate calcium signaling via G proteins (Gi/o types), PLC activation, and subsequent IP3 production, although the sensitivity to pharmacological inhibitors varied from complete inhibition to partial inhibition depending on cell type and lysolipid. Furthermore, we observed that Ki16425 completely inhibited an LPS-induced Ca2+ response in three cell types, but that the effect of VPC32183 varied from complete inhibition in BMMC and C6 glioma cells to partial inhibition in HCT116 cells. Therefore, we conclude that LPS increases [Ca2+]i through Ki16425/VPC32183-sensitive G protein-coupled receptors (GPCR), G protein, PLC, and IP3 in mouse BMMC, rat C6, and human HCT116 cells.

Transcriptional and post-transcriptional mechanisms for lysophosphatidic acid-induced cyclooxygenase-2 expression in ovarian cancer cells

Emerging evidence suggests that lysophosphatidic acid (LPA) is a physiological regulator of cyclooxygenase-2 (Cox-2) expression. Herein we used ovarian cancer cells as a model to investigate the molecular mechanisms that link the LPA G protein-coupled receptors (GPCRs) to Cox-2 expression. LPA stimulated Cox-2 expression and release of prostaglandins though the LPA(1), LPA(2), and LPA(5) receptors. The effect of LPA involves both transcriptional activation and post-transcriptional enhancement of Cox-2 mRNA stability. The consensus sites for C/EBP in the Cox-2 promoter were essential for transcriptional activation of Cox-2 by LPA. The NF-kappaB and AP-1 transcription factors commonly involved in inducible Cox-2 expression were dispensable. Dominant-negative C/EPBbeta inhibited LPA activation of the Cox-2 promoter and expression. Furthermore, LPA stimulated C/EBPbeta phosphorylation and activity through a novel mechanism integrating GPCR signals and a permissive activity from a receptor tyrosine kinase (RTK). This role of RTK was not consistent with LPA activation of C/EBP through transactivation of RTK, as full activation of RTKs with their own agonists only weakly stimulated C/EBP. In addition to the transcriptional activation, the RNA stabilization protein HuR bound to and protected Cox-2 mRNA in LPA-stimulated cells, indicating an active role for HuR in sustaining Cox-2 induction during physiological responses.

Subtype-specific residues involved in ligand activation of the endothelial differentiation gene family lysophosphatidic acid receptors

Lysophosphatidic acid (LPA) is a ligand for three endothelial differentiation gene family G protein-coupled receptors, LPA(1-3). We performed computational modeling-guided mutagenesis of conserved residues in transmembrane domains 3, 4, 5, and 7 of LPA(1-3) predicted to interact with the glycerophosphate motif of LPA C18:1. The mutants were expressed in RH7777 cells, and the efficacy (E(max)) and potency (EC(50)) of LPA-elicited Ca(2+) transients were measured. Mutation to alanine of R3.28 universally decreased both the efficacy and potency in LPA(1-3) and eliminated strong ionic interactions in the modeled LPA complexes. The alanine mutation at Q3.29 decreased modeled interactions and activation in LPA(1) and LPA(2) more than in LPA(3). The mutation W4.64A had no effect on activation and modeled LPA interaction of LPA(1) and LPA(2) but reduced the activation and modeled interactions of LPA(3). The R5.38A mutant of LPA(2) and R5.38N mutant of LPA(3) showed diminished activation by LPA; however, in LPA(1) the D5.38A mutation did not, and mutation to arginine enhanced receptor activation. In LPA(2), K7.36A decreased the potency of LPA; in LPA(1) this same mutation increased the E(max). In LPA(3), R7.36A had almost no effect on receptor activation; however, the mutation K7.35A increased the EC(50) in response to LPA 10-fold. In LPA(1-3), the mutation Q3.29E caused a modest increase in EC(50) in response to LPA but caused the LPA receptors to become more responsive to sphingosine 1-phosphate (S1P). Surprisingly micromolar concentrations of S1P activated the wild type LPA(2) and LPA(3) receptors, indicating that S1P may function as a weak agonist of endothelial differentiation gene family LPA receptors.

Lysophospholipid signaling: beyond the EDGs

As our understanding of the myriads of biological effects caused by lysophospholipids expands, we become witnesses to another miracle of nature that has endowed the simplest lysophospholipids with functions seemingly ubiquitous to every mammalian cell. A decade after the discovery of the EDG family lysophospholipid receptors, the field has gained unimaginable impetus explaining the biological effects of sphingosine-1-phosphate and lysophosphatidic acid (LPA). The discovery of LPA receptors in the purinergic G-protein-coupled receptor (GPCR) gene cluster refined this picture and added complexity to our concepts of lysophospholipid cell signaling. The intracellular lysophospholipid targets - identified and not yet identified - make us realize the dual mediator and second messenger roles of lysophospholipids. In this paper we provide new data obtained concerning LPA-elicited responses using cell lines naturally lacking or intentionally knocked out of many of the known LPA GPCR, widely used by investigators in the field as cells with LPA receptor "null background." Our observations raise caution about the lack of LPA responsiveness in these cells and underline the unprecedented complexity and redundancy of lysophospholipid-evoked cellular responses.

Lysophosphatidic acid decreases the nuclear localization and cellular abundance of the p53 tumor suppressor in A549 lung carcinoma cells

Lysophosphatidic acid (LPA) is a bioactive lipid that promotes cancer cell proliferation and motility through activation of cell surface G protein-coupled receptors. Here, we provide the first evidence that LPA reduces the cellular abundance of the tumor suppressor p53 in A549 lung carcinoma cells, which express endogenous LPA receptors. The LPA effect depends on increased proteasomal degradation of p53 and it results in a corresponding decrease in p53-mediated transcription. Inhibition of phosphatidylinositol 3-kinase protected cells from the LPA-induced reduction of p53, which implicates this signaling pathway in the mechanism of LPA-induced loss of p53. LPA partially protected A549 cells from actinomycin D induction of both apoptosis and increased p53 abundance. Expression of LPA(1), LPA(2), and LPA(3) receptors in HepG2 hepatoma cells, which normally do not respond to LPA, also decreased p53 expression and p53-dependent transcription. In contrast, neither inactive LPA(1) (R124A) nor another G(i)-coupled receptor, the M(2) muscarinic acetylcholine receptor, reduced p53-dependent transcription in HepG2 cells. These results identify p53 as a target of LPA action and provide a new dimension for understanding how LPA stimulates cancer cell division, protects against apoptosis, and thereby promotes tumor progression.

Discovery of potent LPA2 (EDG4) antagonists as potential anticancer agents

The LPA(2) protein is overexpressed in many tumor cells. We report the optimization of a series of LPA(2) antagonists using calcium mobilization assay (aequorin assay) that led to the discovery of the first reported inhibitors selective for LPA(2). Key compounds were evaluated in vitro for inhibition of LPA(2) mediated Erk activation and proliferation of HCT-116 cells. These compounds could be used to evaluate the benefits of LPA(2) inhibition both in vitro and in vivo.

The lysophosphatidic acid 2 receptor mediates down-regulation of Siva-1 to promote cell survival

Lysophosphatidic acid (LPA) promotes cell survival through the activation of G protein-coupled LPA receptors. However, whether different LPA receptors activate distinct anti-apoptotic signaling pathways is not yet clear. Here we report a novel mechanism by which the LPA(2) receptor targets the proapoptotic Siva-1 protein for LPA-dependent degradation, thereby attenuating Siva-1 function in DNA damage response. The carboxyl-terminal tail of the LPA(2) receptor, but not LPA(1) or LPA(3) receptor, specifically associates with the carboxyl cysteine-rich domain of Siva-1. Prolonged LPA stimulation promotes the association of Siva-1 with the LPA(2) receptor and targets both proteins for ubiquitination and degradation. As a result, adriamycin-induced Siva-1 protein stabilization is attenuated by LPA in an LPA(2)-dependent manner, and the function of Siva-1 in promoting DNA damage-induced apoptosis is inhibited by LPA pretreatment. Consistent with this result, inhibition of the LPA(2) receptor expression increases Siva-1 protein levels and augments adriamycin-induced caspase-3 cleavage and apoptosis. Together, these findings reveal a critical and specific role for the LPA(2) receptor through which LPA directly inactivates a critical component of the death machinery to promote cell survival.

Regulation of LPA receptor function by estrogens

17beta-Estradiol induced LPA(1) receptor desensitization in C9 cells stably expressing LPA(1) receptors and transiently expressing estrogen receptor alpha. Such desensitization was evidenced by a reduction in lysophosphatidic acid-mediated Ca(2+)mobilization and it was associated to receptor phosphorylation and internalization. These effects of 17beta-estradiol were rapid (taking place over 5 min) and were blocked by the estrogen receptor antagonist ICI 182780. Similarly, inhibitors of phosphoinositide 3-kinase (wortmannin and LY294002) and of protein kinase C (staurosporine and Gö 6976) blocked 17beta-estradiol-induced LPA(1) receptor desensitization and phosphorylation. Confocal microscopy evidenced LPA(1) receptor internalization in response to 17beta-estradiol treatment. Association between LPA(1) receptors and protein kinase C alpha was suggested by co-immunoprecipitation assays. Protein kinase C alpha was associated with LPA(1) receptors in the absence of stimulus and such association further increased in a dynamic fashion in response to 17beta-estradiol. The results demonstrated that in C9 cells estrogens modulate LPA(1) action through estrogen receptor alpha with the participation of protein kinase C alpha and phosphoinositide 3-kinase.

Validation of an autotaxin enzyme immunoassay in human serum samples and its application to hypoalbuminemia differentiation

BACKGROUND

Autotaxin (ATX), a tumor cell motility-stimulating factor, regulates the blood concentrations of lysophosphatidic acid (LPA), an important and multi-functional bioactive lipid, through its lysophospholipase D activity (lysoPLD). The introduction of ATX measurements into clinical laboratory testing is urgently needed.

METHODS

Anti-human ATX monoclonal antibodies were produced by immunization of recombinant human ATX expressed in a baculovirus system. An immunoassay for the quantitative determination of ATX was established, and human serum samples were assayed.

RESULTS

The within-run and between-run precision, interference, detection limit, and linearity studies were satisfactory. The central 95 percentile reference interval for the serum ATX antigen concentration in healthy subjects was 0.468-1.134 mg/l (n=120) and was strongly correlated with the serum lysoPLD activity. The ATX concentration was significantly (p<0.001) higher in women (0.625-1.323 mg/l) than in men (0.438-0.914 mg/l). The serum ATX concentrations were increased in patients with chronic liver diseases and decreased in postoperative prostate cancer patients but were not altered in nephrosis patients. Thus, serum ATX antigen concentrations could be used to discriminate these hypoalbuminemia conditions.

CONCLUSIONS

The present ATX antigen assay may be useful for clinical laboratory testing.

Quantitative expression of lysophosphatidic acid receptor 3 gene in porcine endometrium during the periimplantation period and estrous cycle

Lysophosphatidic acid (LPA) belongs to the group of lipid messengers, which act via lysophosphatidic acid receptor 3 coupled to G-proteins. The participation of LPA3 in reproductive biology was revealed in mice and has not been studied in gilts. The present study was performed to evaluate the gene expression of LPA3 by a quantitative real-time PCR technique in the endometrium during different stages of pregnancy (days 6-30) and corresponding days of the estrous cycle (days 2-20) as well as in periimplantation period in pigs with surgically detached uterine horns. Based on the most conserved segments of human and rodent LPA3 we obtained a product containing 619bp (GenBank: EF137953), which exhibited high homology with human and rodents sequences. The highest transcript level was noted on days 10-12 of gestation in comparison to remaining periods and during pregnancy on days: 6-7, 8-9, 10-12 and 13-14 in comparison with the corresponding days of the estrous cycle. Higher mRNA level was noted in the horn containing embryos compared to the contralateral horn, where embryos did not develop. The results imply the important role of receptor LPA3 during early pregnancy.